The Terrestrial Kingdom is the middle of three "degrees of glory," in which the highest or Celestial Kingdom is compared to the sun, the middle or Terrestrial Kingdom is compared to the moon, and the lowest or Telestial Kingdom is compared to the stars. Also, "in addition to the degrees of glory, there is a place of no glory, called perdition, reserved for those who commit the unpardonable sin." (Bible Dictionary: Degrees of Glory)
Persons who will attain the Terrestial Kingdom in the afterlife, according to the Church's teachings, include those who did not know about Jesus Christ during their life, but accepted Christianity after being taught about it in the afterlife.